Add unsetValueOption() to the Form\Element\Select and Form\Element\MultiCheckbox

It could be nice to have such a method to remove a value option from a form element in runtime.

Instead of this in the user land:
```
$valueOptions = $element->getValueOptions();
if (isset($valueOptions['something'])) {
    unset($valueOptions['something']);
    $element->setValueOptions($valueOptions);
}
```

just this:
```
$element->unsetValueOption('something');
```